1. Field of the Invention
The present invention relates to an electrostatic image developing toner suitable for use in electrophotography printing, electrostatic printing or the like; and a novel urethane-modified polyester resin used for the toner. More specifically, this invention relates to an electrostatic image developing toner having good charge properties, blocking resistance and offset resistance as well as excellent resistance against sticking to a heated roll and wax dispersibility; and a novel urethane-modified polyester resin used for it.
2. Description of the Related Art
With the progress of office automation, a demand for copying machines or laser printers making use of electrophotography has shown a rapid increase and at the same time, a requirement for their performances has come to be severer. In order to obtain a visible image by electrophotography, a generally adopted process is to electrify a photo conductor such as selenium, amorphous silicon or organic semiconductor, expose it to light, develop an image by using a toner-containing developer, transfer the toner image, which has been formed on the photo conductor, to a printed body such as paper and then fix the image by using a heated roll or the like. It is needless to say that the developed image must be vivid, being free from fogging and having a sufficient image density. In recent days, however, it has strongly been requested to carry out fixation at a lower temperature, from the viewpoint of speeding up, energy saving and improvement of safety and at the same time, to provide a toner having excellent low-temperature fixing properties. For improving the fixing properties of a toner, it is generally necessary to lower the melt viscosity of the toner, thereby increasing the adhesion area with a base material to be fixed. For this purpose, the glass transition point (Tg) or molecular weight of the conventionally used binder resin is lowered. Such a countermeasure is however accompanied with the drawbacks that stable maintenance of the particle condition of the toner upon usage or storage is not easy, because a resin having a low glass transition point is usually inferior in blocking resistance; and upon fixation of the toner image by the heated-roll fixing method, the heated roll is directly brought into contact with the toner under a molten state and so-called offset phenomenon occurs, in other words, a portion of the toner transferred onto the heated roll smears a printed body such as paper or the like to be fed next. Such a tendency becomes apparent when the molecular weight of the resin is smaller.
Recently, a digital system which can output information from a computer or facsimile has drawn attentions as an electrophotographic image forming system. Since laser is used as exposure means in this digital system, a toner having a small particle size is required so as to permit the output of a more minute line and therefore a more delicate image compared with an analog system. When the particle size of a toner is reduced, however, the wax having poor dispersibility tends to fall off and be recovered as a dust. In general, the dust which has appeared in the pulverizing step is recovered and recycled as the raw material. A large wax content in the dust makes recycling difficult and is therefore not preferred. Moreover, the wax falls off from the surface of the toner during stirring with a carrier in a copying machine, by which the charge amount becomes unstable or filming occurs on a photo conductor. It is preferred to use the wax after being dispersed in the toner minutely and uniformly. As the wax, low-molecular-weight polypropylene or polyethylene is generally employed, but owing to poor compatibility with polyester or the like, the wax tends to have a large particle size in the dispersed form.
Various toners using, as a binder resin, a polyester resin instead of a styrene acrylic resin or epoxy resin are proposed in consideration of charge properties or fixing properties (ex. Japanese Patent Laid-Open No. 284771/1986, Japanese Patent Laid-Open No. 291668/1987, Japanese Patent Publication No. 101318/1995, Japanese Patent Publication No. 3663/1996, U.S. Pat. No. 4,833,057 and the like). The use of a polyester resin is however accompanied with the problem that its surface tension is greater than that of a styrene acrylic resin frequently employed as a binder resin so that winding of paper and the like onto a heated roll tends to occur. The use of a polyester resin as a binder resin, on the other hand, is also accompanied with the problem that although the negative charge properties of the resulting toner becomes higher in proportion to the concentration of the terminal carboxyl group, an increase in the concentration of the terminal carboxyl group causes lowering of moisture resistance and an electrified amount of the toner decreases, influenced by the water content at the time of high humidity, leading to a deterioration in the quality of the developed image.
Furthermore, since treatment of a copying machine including that of a toner is requested to be maintenance free, there is an eager demand for a toner which can provide a developed image stably for a long period of time.